[Ada] Use Actions field of freeze nodes for subprograms

[Ada] Use Actions field of freeze nodes for subprograms

This has a couple of advantages: 1) the actions are analyzed with checks
disabled and 2) they are considered elaboration code by Sem_Elab.

gcc/ada/

	* exp_ch13.adb (Expand_N_Freeze_Entity): Delete freeze nodes for
	subprograms only if they have no actions.
	* exp_ch6.adb (Freeze_Subprogram): Put the actions into the Actions
	field of the freeze node instead of inserting them after it.
	* sem_elab.adb (Is_SPARK_Semantic_Target): Fix typo in comment.
	* gcc-interface/trans.cc (process_freeze_entity): Return early for
	freeze nodes of subprograms with Interface_Alias set.